A signal generated by a transmitting antenna may cause interference for a nearby receiving antenna, i.e., a so-called “victim” antenna, which may inhibit the victim's ability to receive signals. Specifically, when a transmission antenna is configured to transmit a main signal to a target antenna, sidelobes of the main signal may extend in a variety of directions, and in particular, in the direction of the victim. The signal received by the victim antenna depends on the transmitted power level, the gain of the sidelobe level of the transmission antenna, the separation between the transmission and victim antennas, and the characteristics of the victim receiving antenna. The sidelobes of the transmission antenna have an angular dependence, so that changes in the transmission antenna's target antenna location results in changes in the signal level received by the victim antenna. Thus, the interference received by the victim antenna may vary with time and may exceed an interference level that disrupts the operation of the victim antenna.
Several prior art systems for reducing unintentional interference with victim antenna reception have been developed. Such systems typically involve either modifying the transmission antenna so as to reduce the presence of sidelobes, or modifying the victim antenna so as to cancel received interference. One example of a transmitter-side modification is disclosed in U.S. Pat. No. 5,781,845 to Dybdal et al. That patent discloses a system including several antenna elements and adaptive weighting circuitry, which adjusts weighting coefficients during transmission. That patent addresses an adaptive antenna design having broad coverage requirements that illuminates the surrounding terrain, resulting in reflected signal components referred to as multipath that may interfere and degrade reception of the signal received by a target receiver. The antenna elements also sample the reflected multipath signals, which are cross-correlated with the transmitted signal. This cross-correlation is minimized so as to reduce the transmitted strength of an interfering, multipath signal. However, when the transmission antenna is more directive than the broad coverage antenna described in U.S. Pat. No. 5,781,845, the antenna system may be installed in a fixed location whose siting is specifically examined and selected to minimize multipath.
An example of a receiver-side modification is disclosed in U.S. Pat. No. 5,440,308 to Dybdal et al., which discloses a main antenna that receives a desired signal through a main beam, and interference at an angle different than that of the main beam. Auxiliary antennas also receive the interference signals, and the output of the main antenna is cross-correlated with the elements of the auxiliary antenna. This cross-correlation is minimized so as to reduce the interference signal received by the main antenna. However, such receiver-side modifications may be expensive or complicated to implement, and further require positive action by the owner/operator of the victim antenna. In addition, such receiver-side modifications result in creating pattern nulls in the victim's antenna that may undesirably reduce the victim's antenna coverage.